Perfluorofatty acids and derivatives thereof are widely used as emulsifiers in producing a fluoroelastomer by emulsion polymerization, and ammonium perfluorooctanoate (C7F15COONH4) is used most frequently, among others. Although it is excellent in performance in emulsion polymerization, ammonium perfluorooctanoate is expensive and readily remains in product polymers in the step of coagulation, prolonging the cure time of the resulting curable rubber compositions and/or leading to inferior characteristics of final cured products; a step for sufficient washing is thus required prior to drying.
Patent Document 1 describes C6F13CH2CH2SO3M as an emulsifier capable of substituting for ammonium perfluorooctanoate. However, it causes changes in polymers probably due to the chain transfer reaction involving the methylene groups; hence the range of use thereof is limited.
Patent Document 2 described the use of CF3(CF2)4COO− (NH4)+ in producing tetrafluoroethylene/propylene copolymer latexes. However, it does not describe anything about the production of vinylidene fluoride-containing fluoroelastomers.
Patent Document 3 describes the use of potassium or sodium perfluorohexanoate as an emulsifier to be used in emulsion polymerization of hexafluoropropylene and vinylidene fluoride. However, it is not used in any example; what problem or problems will be encountered is not known.    [Patent Document 1] Japanese Kohyo Publication (laid open under PCT) 2004-509993    [Patent Document 2] Japanese Kokai (Laid-open) Publication 2006-321797    [Patent Document 3] British Patent No. 823974